Conventional hydronic distribution systems utilized in many buildings include a variable speed drive that relies on differential pressure measurements within the system to control the speed for one or more variable speed pumps that supply fluid to a chiller or boiler. For example, FIG. 1 shows a prior-art, closed-loop, hydronic, distribution system 10 having a chiller/boiler 12 in fluid communication with two variable speed pumps 14. In turn, the pumps 14 are electronically controlled by a variable speed drive 16 that receives differential pressure information 18 from one or more differential pressure sensors 20. The sensed pressure is the pressure difference between a first point 22 upstream of at least one of the coils 24 and a second point 26 downstream of at least one of the self-regulating valves 28. If the hydronic distribution system 10 operates as a heating system, then hot fluid from the boiler 12 proceeds to cooling coils 24, and conversely if the hydronic distribution system 10 operates as a cooling system, then cool fluid from the chiller 12 proceeds to warming coils 24. A flow controlled bypass 29 valve may also be included to divert fluid from the discharge of the pump(s) to the return line of the chiller or boiler in order to maintain minimum flow through the boiler or chiller at all times.
The pressure sensor 20 typically monitors the differential pressure across the supply and return header 30, but other points of differential pressure measurement are sometimes used, and the pressure information is utilized by the variable speed drive 16 to control the speed of the pumps 14 to maintain the differential pressure within a desired range of a predetermined differential pressure set-point for across the header 30. The pressure set-point is established so that the system 10 will satisfy the required amount of process fluid flow at all loads under all operating conditions.
Control of the pumps to maintain the pressure set-point may be based on a proportional and integral (PI) or proportional, integral and derivative (PID) control loop theory. In typical operation, the monitored differential pressure decreases as the valves 28 open showing more demand for process fluid in the system 10. As a result, the pumps 14 speed up to maintain the set-point pressure and provide the required fluid. If the valves 28 begin to close, then the differential pressure increases, which in turn causes the pumps 14 to slow down in order to maintain the set-point pressure. The illustrated system 10 and associated distribution method using differential pressure is considered the standard method of fluid control through a hydronic distribution system.